Switching regulators are intended to be efficient machines for converting an input voltage to an output voltage. The two most common types of switching regulators are Boost (voltage increasing converters) and Buck (voltage decreasing regulators). Both Boost and Buck regulators are very important for battery powered applications such as cellphones. As shown in FIG. 1A, a traditional implementation for a Buck regulator includes a switch M1 connected between an input voltage (VBATT in this case) and a node Vx. A diode D is connected between the node Vx and ground. An inductor L is connected between Vx and the output node (VOUT) of the regulator. A filtering capacitor connects VOUT to ground. The node VOUT is also connected to a load (not shown).
A control circuit turns switch M1 ON and OFF in a repeating pattern. This causes the Buck regulator to have two distinct operational phases. In the first phase, shown in FIG. 1B, the switch M1 is ON. During this phase, called the charging phase, the inductor is connected between the battery and the output node VOUT. This causes current to flow from the battery to the load. In the process energy is stored in the inductor L in the form of a magnetic field.
In the second, or discharge phase the switch M1 is opened (see FIG. 1C). In this phase the diode and inductor are connected in series between ground and the load. In this phase, current supplied by the inductor's magnetic field flows to the output node VOUT and the load. As the inductor's magnetic field collapses and the voltage over the inductor falls, the diode prevents current flowing through the inductor from reversing direction and flowing from the load to ground.
In general, switching regulators work in environments where both the input and output voltage are dynamic voltages. Input voltages change as battery voltages decline over time or as other components draw more power. Output voltages change depending on load requirements. Switching regulators react to changes in input and output voltages by varying the amount of time that the switch M1 remains ON. This is done using two different methods. In the first method, the switching frequency is varied—as the load on the regulator increases (relative to its supply) the switching frequency is increased. This is known as pulse frequency modulation or PFM. In the second method a fixed switching frequency is used and the amount of time that the switch M1 is turned ON is varied. For larger loads, the switches stay ON longer. This is known as pulse width modulation of PWM. Of the two methods, PWM is often preferred because it produces noise at a known and therefore filterable fixed frequency. Filtering the noise created by a PFM regulator can be problematic—especially in portable applications.
The regulator architecture just illustrated suffers one fundamental flaw: the diode D has, by nature a forward voltage drop. Depending on the type of diode, this can be fairly small, but is still generally unacceptable for low voltage applications. For this reason, it is common to replace the diode D with a second switch M2. FIGS. 2A and 2B show Boost and Buck regulators of this type, respectively. The basic idea is that the switch M2 operates with no voltage drop (when switched ON) overcoming the disadvantages inherent in diode based designs.
In regulators of this type, the switch M1 is often referred to as the high-side switch and the switch M2 is referred to as the low-side switch. The switch M2 is also referred to as a “synchronous rectifier” because the two switches are driven synchronously—when one is ON, the other is OFF. In the real world, this is never quite the case. It takes time to turn the switches ON and OFF and control cannot be done with absolute precision. For this reason, the act of turning a switch OFF is always done slightly in advance of the act of turning the other switch ON. This technique, known as break-before-make or BBM avoids the situation where both switches are ON at the same time and power is connected to ground (a condition known as shoot through).
In many switching regulators, the high and low-side switches are fabricated as MOSFET devices that are integrated monolithically with the control circuit. During the time between switching OFF the low-side switch and the switching ON of the high-side switch, when the channels of both high and low side MOSFET devices are not conducting, the inductor current forward biases the body diode in the low side MOSFET switch. This is undesirable for the following reasons:
1) Minority carriers are injected into the substrate (on which the MOSFET devices are fabricated) which may upset other circuits controlling the power devices;
2) The forward biased body diode must be reversed before the high side switch can fully conduct; and
3) The larger voltage drop across the body diode compared to the voltage drop across the channel is less efficient.